Interaction of cytochrome c with liposomes: covalent labeling of externally bound protein by the fluorescent probe, azidonaphthalenedisulfonic acid, enclosed in the inner aqueous compartment of unilamellar vesicles

Assigning membrane binding geometry of cytochrome C by polarized light spectroscopy

In this work we demonstrate how polarized light absorption spectroscopy (linear dichroism (LD)) analysis of the peptide ultraviolet-visible spectrum of a membrane-associated protein (cytochrome (cyt) c) allows orientation and structure to be assessed with quite high accuracy in a native membrane environment that can be systematically varied with respect to lipid composition. Cyt c binds strongly to negatively charged lipid bilayers with a distinct orientation in which its alpha-helical segments are on average parallel to the membrane surface. Further information is provided by the LD of the pi-pi( *) transitions of the heme porphyrin and transitions of aromatic residues, mainly a single tryptophan. A good correlation with NMR data was found, and combining NMR structural data with LD angular data allowed the whole protein to be docked to the lipid membrane. When the redox state of cyt c was changed, distinct variations in the LD spectrum of the heme Soret band were seen corresponding to changes in electronic transition energies; however, no significant change in the overall protein orientation or structure was observed. Cyt c is known to interact in a specific manner with the doubly negatively charged lipid cardiolipin, and incorporation of this lipid into the membrane at physiologically relevant levels was indeed found to affect the protein orientation and its alpha-helical content. The detail in which cyt c binding is described in this study shows the potential of LD spectroscopy using shear-deformed lipid vesicles as a new methodology for exploring membrane protein structure and orientation.

Ordering Transitions in Liquid Crystals Permit Imaging of Spatial and Temporal Patterns Formed by Proteins Penetrating into Lipid-Laden Interfaces

Recent studies have reported that full monolayers of L-α-dilaurylphosphatidylcholine (L-DLPC) and D-α-dipalmitoylphosphatidylcholine (D-DPPC) formed at interfaces between thermotropic liquid crystals (LCs) and aqueous phases lead to homeotropic (perpendicular) orientations of nematic LCs and that specific binding of proteins to these interfaces (such as phospholipase A₂ binding to D-DPPC) can trigger orientational ordering transitions in the liquid crystals. We report on the nonspecific interactions of proteins with aqueous-LC interfaces decorated with partial monolayer coverage of L-DLPC. Whereas nonspecific interactions of four proteins (cytochrome c, bovine serum albumin,immunoglobulins, and neutravidin) do not perturb the ordering of the LC when a full monolayer of L-DLPC is assembled at the aqueous-LC interface, we observe patterned orientational transitions in the LC that reflect penetration of proteins into the interface of the LC with partial monolayer coverage of L-DLPC. The spatial patterns formed by the proteins and lipids at the interface are surprisingly complex, and in some cases the protein domains are found to compartmentalize lipid within the interfaces. These results suggest that phospholipid-decorated interfaces between thermotropic liquid crystals and aqueous phases offer the basis of a simple and versatile tool to study the spatial organization and dynamics ofprotein networks formed at mobile, lipid-decorated interfaces.

Coulombic and noncoulombic effect of polyanions on cytochrome c structure

The properties of the complexes of ferricytochrome c with two different polyanions--poly(vinylsulfate) and poly(4-styrene-sulfonate)--with a comparable charge density but with the different size of the uncharged part of their molecules have been studied by means of optical spectroscopy, differential scanning colorimetry, and gel chromatography. Ferriccytochrome c formed a complex with the former one through coulombic interactions and remained in a native-like state. The addition of the second polyanion to a solution of ferric cytochrome c at a low ionic strength, pH 7.0, resulted in profound conformational change in the hydrophobic core of protein (opening of the heme crevice with a perturbation of the methionine 80-heme iron bond and the hydrophobic core of the protein). These may be understood as an involvement of noncoulombic (hydrophobic, H-bonding) interactions of the uncharged part of the polyanion molecule. Conformational changes and the observed shift in acidic transition from low spin to high spin state of ferric cytochrome c detected in the presence of the polyanions may have biological implication in understanding the origin of conformational changes in proteins induced in the course of their interaction with membrane lipids and membrane proteins.

Kinetics of radiation- and cytochrome c-induced modifications in liposomes analysed by FT-Raman spectroscopy

Fourier transform Raman spectroscopy on artificial lipid membranes was used to study radiation-induced peroxidation processes as a function of time after radiation exposure. The time dependent intensity changes of the Raman lines of various C=C bondings were compared to results obtained by measuring conjugated dienes and by the thiobarbituric acid test for malondialdehydes. The results show that mainly the cis C=C bonds of the lipid chains are involved and, therefore, indicate that gamma-radiation induces conformational changes in the lipid chain while the mobility of the lipid chains is reduced. New Raman bands can be assigned to aldehyde products induced at the end of the peroxidation process. The immediate decrease of the =CH vibration lines was directly correlated with the formation of conjugated C=C double bonds suggesting that these vibration lines are in contrast to the C=C lines solely Raman active, when isolated C=C bonds are present. Cytochrome c (ox.) incorporated into the bilayer of the artificial membranes induced autooxidation processes not influenced by gamma-radiation. It was observed that cytochrome c (ox.)-induced changes of the relative intensity of the C=C bonds differ from those induced by gamma-radiation. These results of cytochrome c together with the inhibitory effects of the antioxidant alpha-tocopherol suggest that the radical species involved in the cytochrome c induced process might be different from the free radicals involved in the gamma-radiation-induced process.

Interaction of horse heart cytochrome c with lipid bilayer membranes: effects on redox potentials

Cyclic voltammetry has been used to study the effects of interactions between horse cytochrome c and solid-supported planar lipid membranes, comprised of either egg phosphatidylcholine (PC) or PC plus 20 mol.% cardiolipin (CL), on the redox potential and the electrochemical electron transfer rate between the protein and a semiconductor electrode. Experiments were performed over a wide range of cytochrome c concentrations (0-440 microM) at low (20 mM) and medium (160 mM) ionic strengths. Three types of electrochemical behavior were observed, which varied as a function of the experimental conditions. At very low cytochrome c concentration (approximately 0.1 microM), and under conditions where electrostatic forces dominated the protein-lipid membrane interaction (i.e., low ionic strength with membranes containing CL), a redox potential (approximately 265 mV) and an electrochemical electron transfer rate constant (0.09 s[-1])were obtained which compare well with those measured in other laboratories using a variety of different chemical modifications of the working electrode. Two other electrochemical signals (not reported with chemically modified electrodes) were also observed to occur at higher cytochrome c concentrations with this membrane system, as well as with two other systems (membranes containing CL under medium ionic strength conditions, and PC only at low ionic strength). These involved positive shifts of the cytochrome c redox potential (by 40 and 60 mV) and large decreases in the electron transfer rate (to 0.03 and 0.003 s[-1]). The observations can be rationalized in terms of a structural model of the cytochrome c-membrane interaction, in which association involves both electrostatic and hydrophobic forces and results in varying degrees of insertion of the protein into the hydrophobic interior of the membrane.

Surface plasmon resonance studies of complex formation between cytochrome c and bovine cytochrome c oxidase incorporated into a supported planar lipid bilayer. I. Binding of cytochrome c to cardiolipin/phosphatidylcholine membranes in the absence of oxidase

The mechanism of interaction between cytochrome c and a solid-supported planar phosphatidylcholine membrane containing varying amounts of cardiolipin (0-20 mol%) has been studied over a wide range of protein concentrations (0-450 microM) and ionic strength conditions (10-150 mM), by direct measurement of protein binding using surface plasmon resonance (SPR) spectroscopy. The results demonstrate that cytochrome c binds to such phospholipid membranes in two distinct phases characterized by very different (approximately one order of magnitude) affinity constants. The second phase is dependent upon the prior occurrence of the first binding process. Although the binding affinities for both modes of binding are highly sensitive to both the cardiolipin concentration and the ionic strength of the buffer solution, indicating that electrostatic forces are involved in these processes, binding cannot be reversed by salt addition or by dilution. Furthermore, the final saturation levels of adsorbed protein are independent of ionic strength and cardiolipin concentration. These observations suggest that binding involves more than a simple electrostatic interaction. Invariance in the shapes of the SPR spectra indicates that no major structural transitions occur in the proteolipid membrane due to cytochrome c binding, i.e., the bilayer character of the lipid phase appears to be preserved during these interactions. Based on these results, a model of the lipid membrane-cytochrome c interaction is proposed that involves varying degrees of protein unfolding and subsequent binding to the membrane interior via hydrophobic forces.

Persistence of cytochrome c binding to membranes at physiological mitochondrial intermembrane space ionic strength

We have shown that cytochrome c (cyt c) diffuses primarily in three dimensions in the intermembrane space (IMS) of intact mitochondria at physiological ionic strength (I). Recently, we found that a small percentage (11.2 +/- 2.1%) of endogenous cyt c remains bound to inner mitochondrial membranes (IMM) at high, physiological I (I = 150 mM), even after extensive washing with solutions at physiological I, overnight dialysis, changes in medium osmolarity, or further purification of IMM at high I using self-generating Percoll gradients. Measurements of heme c/heme a ratios, and electron transport (ET) reactions in which cyt c participates, confirmed the presence of a low amount of this I-resistant, membrane-bound form of cyt c (MB-cyt c), that had one third of the ET activity of electrostatically-bound cyt c (EB-cyt c), and which could not account for maximal ET rates. The amount of MB-cyt c was significantly increased above endogenous MB-cyt c by exposing KCl-washed IMM to increasing concentrations of exogenous cyt c. Also, subjecting large unilamellar vesicles (LUV) to successive cycles of cyt c binding/high I KCl-washes gave progressive increases in MB-cyt c. These protocols allowed in vitro characterization of MB-cyt c. The I at which binding takes place affects the affinity of cyt c for membranes, and oxidized cyt c had a greater intrinsic affinity for IMM or SUV than reduced cyt c. MB-cyt c appears to be bound partially by hydrophobic interactions since MB-cyt c was detected on negatively charged (asolectin) LUV and also on neutral, zwitterionic (phosphatidylcholine) LUV at high I. Consistent with the concentration-dependent changes in MB-cyt c, decreasing the IMS-volume of intact mitochondria (i.e., increasing th endogenous IMS-cyt c concentration) by metabolic or osmotic means increased the amount of MB-cyt c. After cyt c was delivered into the IMS by liposome-mediated low pH-induced fusion, resonance energy transfer showed a time-dependent cyt c-membrane proximity which was consistent with slow exchange of soluble IMS-entrapped cyt c molecules with a population bound to membranes at I = 150 mM. We conclude that, even though the majority of functional IMS-cyt c diffuses in three dimensions, a small portion remains firmly bound on the surface of the IMM under I conditions that are physiological for intact mitochondria. The occurrence of MB-cyt c may reflect an intrinsic conformational flexibility in cyt c, that allows a degree of membrane penetration and the formation of hydrophobic interactions which stabilize the membrane-bound form. The persistence of cyt c-membrane interactions under physiological I conditions indicates that cyt c-mediated ET in the IMS involves both fast (3D-diffusion) and slow (2D-diffusion) pathways for electron transfer.

Calorimetric studies of the interactions of cytochrome c with dioleoylphosphatidylglycerol extruded vesicles: ionic strength effects

Cytochrome c has been studied as an example of a peripheral membrane protein which interacts with the lipids as well as the proteins of the inner mitochondrial membrane. In order to elucidate the thermodynamic properties of these interactions, isothermal titration calorimetry and differential scanning calorimetry (DSC) were used to study the binding of cytochrome c to negatively charged dioleoylphosphatidylglycerol (DOPG) extruded vesicles as a function of ionic strength. The binding constant and enthalpy of association decrease with increasing ionic strength, with no binding detected above 0.5 M NaCl. The enthalpy of the binding of cytochrome c to DOPG-extruded vesicles was 15 kcal/mol, and the binding constant was 6 x 10(6) M-1 at the lowest ionic strengths. The minimum size of the lipid cluster to which the protein bound was found to be approx. 9 lipid molecules in the titration calorimetry measurements and as low as 5 lipid molecules in the DSC measurements. The stability of the bound cytochrome c was found to be reduced; the thermal denaturation temperature was lowered from 83 to 50 degrees when bound to DOPG. The results of this study support previous suggestions that cytochrome c may undergo a conformational change when it binds to charged lipids such as DOPG. The results also support the suggestion that the protein penetrates partially into the lipid bilayer.

The interaction of horse heart cytochrome c with phospholipid bilayers. Structural and dynamic effects

The interaction of cytochrome c with phospholipid bilayers is reviewed. Special emphasis is given to the structural and dynamic perturbations induced, either in the membrane lipid component or protein itself, by the lipid-protein interaction. The lipid-induced perturbations in the structure of cytochrome c involve: i) conformational changes in and around the heme crevice, converting the heme iron to a high-spin state: and ii) a destabilisation/loosening of the overall tertiary and secondary structure. This highly mobile, partially unfolded intermediate of cytochrome c has a remarkable resemblance to partially folded membrane-bound intermediates of the precursor protein. The functional implications of lipid-protein intermediates for (apo) cytochrome c in (protein-translocation) electron-transfer are discussed.

Reversible unfolding of cytochrome c upon interaction with cardiolipin bilayers. 1. Evidence from deuterium NMR measurements

Deuterium NMR has been used to investigate the structure and dynamic state of cytochrome c complexed with bilayers of cardiolipin. Reductive methylation was employed to prepare [N epsilon, N epsilon-C2H3]lysyl cytochrome c, and deuterium exchange provided labeling of backbone sites to give [amide-2H]cytochrome c or more selective labeling of just histidine residues in [epsilon-2H]histidine cytochrome c. Deuterium NMR measurements on [N epsilon, N epsilon-C2H3]lysyl cytochrome c in the solid state showed restricted motions, fairly typical of the behavior of aliphatic side-chain sites in proteins. The [amide-2H]cytochrome c provided "immobile" amide spectra showing that only the most stable backbone sites remained labeled in this derivative. Relaxation measurements on the aqueous solution of [amide-2H]cytochrome c yielded a rotational correlation time of 7.9 ns for the protein, equivalent to a hydrodynamic diameter of 4.0 nm, just 0.6 nm greater than its largest crystallographic dimension. Similar measurements on [epsilon-2H]histidine cytochrome c in solution showed that all labeled histidine residues were also "immobile" compared with the overall reorientational motion of the protein. The interaction with cardiolipin bilayers appeared to create a high degree of mobility for the side-chain sites of [N epsilon, N epsilon-C2H3]lysyl cytochrome c and perturbed backbone structure to instantaneously release all deuterons in [amide-2H]cytochrome c. The [epsilon-2H]histidine cytochrome c derivative, when complexed with cardiolipin, failed to produce any detectable wide-line 2H NMR spectrum, demonstrating that the overall reorientational motion of bound protein was not isotropic on the NMR time scale, i.e., tau c greater than 10(-7)s.(ABSTRACT TRUNCATED AT 250 WORDS)

Redox proteins as electron acceptors from chlorophyll triplet state in lipid bilayer vesicles: kinetics of reduction of membrane reconstituted cytochrome c oxidase mediated by 2,5-di-t-butyl benzoquinone and cytochrome c

Laser flash photolysis was used to determine the kinetics of electron transfer between membrane-bound triplet chlorophyll (3C), cytochrome c (cyt c) located in the external water phase, and vesicle-reconstituted cytochrome c oxidase (CCO). 2,5-Di-t-butyl benzoquinone (2,5 TBQ) was used as an electron transfer mediator between 3C and cyt c. A light-induced cyclic electron transfer sequence between the redox components was observed (3C----2.5 TBQ----cyt c----CCO----C+.). Under optimum conditions of membrane surface charge and ionic strength, the overall efficiency of CCO reduction (based on 3C generated by the laser flash) was 14%. Under the anaerobic conditions used, CCO reoxidation (occurring via electron transfer to C+.) was quite slow (halftime approx. 1 s at 75 mM ionic strength). The multicomponent system displayed a high level of stability, as indicated by its ability to undergo many cycles of reduction and reoxidation without any apparent degradation of the components. These results demonstrate the feasibility of constructing complex electron transfer chains, including both soluble and membrane-bound redox proteins, in artificial lipid bilayers, whose properties can be readily controlled by manipulating parameters such as ionic strength and membrane composition.

Interaction of pyridoxal phosphate modified cytochromes c with mitoplasts

1. The stability of the native conformation of the heme crevice of pyridoxal phosphate (PLP)-ferricytochromes c as assayed by the pK, for 695 nm absorption band varies considerably. The pKa values are 8.76 for cytochrome c modified by PLP at lysine 79[PLP(Lys 79)-cyt. c], 9.23 for cytochrome c modified by PLP at lysine 86 [PLP(Lys 86)-cyt.c], 9.34 for doubly PLP substituted cytochrome c at lysines 79 and 86 [(PLP)2-cyt. c], 9.50 for triply substituted cytochrome c [(PLP)3-cyt. c] and 9.06 for native cytochrome c, which indicates less stable heme crevice of PLP-cytochrome c. 2. The singly PLP-modified cytochrome c indicate decreased activities with mitochondrial cytochrome c oxidase in the following order: PLP(Lys 86)-cyt. c less than PLP(Lys 79)-cyt. c less than native cytochrome c. The high affinity Km for PLP(Lys 86)-cyt. c, PLP(Lys 79)-cyt. c and native cytochrome c are 0.28 microM, 0.16 microM and 0.02 microM respectively. 3. PLP-cytochromes c show decreased binding affinities to fluorescence probes 12-(9-antroyl)-stearic acid and pyrene-labelled mitoplasts. The quenching of singly PLP-modified cytochrome c depends significantly on the ionic strength.